This invention relates to a method for the production of microcapsules containing hydrophobic capsule core material and more particularly to an improved method for the production of microcapsules each having a polyurea capsule wall which are useful for the manufacture of pressure-sensitive copying paper.
Microcapsules find their usefulness in various fields such as pressure-sensitive copying papers, heat-sensitive recording papers, adhesives, fertilizers, pharmaceuticals, foods, cosmetics, etc. There are known various method for making microcapsules useful for those purposes. Among them, there are included the "coacervation" method, the "interfacial polymerization" method and the "in situ polymerization" method. The most typical capsule wall material in the "coacervation" method is one of the gelatin-gum arabic type which is of a natural polymer material. On the other hand, recently microcapsules each having a synthetic film capsule wall produced by the "interfacial polymerization" method or by the "in situ polymerization" method have been put to practical use as well. Among the materials for such synthetic film capsule walls, there are included various combinations, e.g., those of isocyanate and water, isocyanate and polyamine, isocyanate and polyol, isothiocyanate and water, isothiocyanate and polyamine, isothiocyanate and polyol, urea and formaldehyde, melamine and formaldehyde, and, acid chloride and amine, in addition to materials for epoxy resin. In comparison with microcapsules having capsule walls formed by a natural polymer material, microcapsules having synthetic film capsule walls are especially advantageous in the manufacture of pressure-sensitive copying paper in that the coating composition including the microcapsules can be prepared in high concentration and accordingly the coating operation can be carried out at a high speed, in that, since the capsule wall structure formed is relatively of high density, oily droplets enclosed in the microcapsules are substantially prevented from natural exudation (this is particularly advantageous in the manufacture of the so-called "self-contained" type pressure-sensitive copying paper in which both the color former microcapsules and the color acceptor are coated on a single surface of a paper sheet), in that they are easy and not expensive to prepare, and in that they have a good resistance to moisture.
However, among microcapsules having synthetic film capsule walls, those utilizing isocyanate compounds are disadvantageous in that they are not so good in the solvent resistance, that, when they are placed in an atmosphere including a solvent, oily droplets in the microcapsules tend to be extracted to come into contact with the color acceptor to incidentally and undesirably develop a color and in that they are easily ruptured, while no handwriting or typewriting force is applied, by incidental and undesirable friction forces applied as by rubbing to produce smudges by color developing.
The principal object of the invention is to provide a novel and improved method for the production of microcapsules each having a synthetic film capsule wall in which the microcapsules obtained have a good resistance against rubbing friction and a good solvent resistance without sacrificing the advantages usually obtained with those microcapsules of such kind.
Another object of the invention is to provide an improved method for the production of microcapsules each having a synthetic film capsule wall in which the microcapsules produced have generally uniform particle sizes.
A further object of the invention is to provide an improved method for the production of microcapsules each having a polyurea film capsule wall which are useful for the manufacture of pressure-sensitive copying paper.
The other objects and advantages of the invention will become apparent from the following detailed description.